A copper foil is used in a rigid printed circuit board, a flexible printed circuit board, an electromagnetic wave shield material, acurrent collector of a cell, and other various fields.
Among these fields, in the field of printed circuit boards (flexible circuit boards, hereinafter referred to as “FPCs”) which are bonded to a polyimide film, in hard disk (hereinafter, referred to as “HDD”) suspension materials or tape automated bonding (hereinafter, referred to as “TAB”) materials, improvement of the strength of the copper foil has been requested.
The suspensions which are mounted on HDDs have mostly changed from the conventionally used wire type suspensions to the wiring-integrated type suspensions in which buoyancy and positional accuracy of the flying head to the storage medium of the disk are stable along with the increase of the capacity of HDDs.
This wiring-integrated type suspension includes the following three types.    a. A so-called FSA (flex suspension aggregation) type obtained by working a flexible printed board and using a binder for bonding    b. A so-called CIS (circuit integrated suspension) type obtained by shaping a precursor of a polyimide resin, amic acid, then imidizing it and further plating the polyimide to thereby form wiring.    c. A so-called TSA (trace suspension aggregation) type obtained by etching a laminate comprised of stainless steel foil, polyimide resin, and copper foil to thereby work it to a predetermined shape
A TSA suspension enables easy formation of a flying lead by laminating a copper alloy foil having a high strength and has a high degree of freedom in shaping, is relatively cheap, and is good in dimensional precision, so is being widely used.
A laminate formed according to the TSA method is produced by using as materials a stainless steel foil of a thickness of about 12 to 30 μm, a polyimide layer of a thickness of about 5 to 20 μm, and a copper alloy foil of a thickness of about 7 to 14 μm.
In the production of the laminate, first, a polyimide resin solution is coated on the base member of stainless steel foil. After coating, the solvent is removed by preheating, then heat treatment is further carried out for imidization. Then, the copper alloy foil is superimposed on the imidized polyimide resin layer and the aggregation is hot pressed at a temperature of about 300° C. for lamination to thereby produce a laminate comprised of a stainless steel layer, polyimide layer, and copper alloy layer.
At the time of this heating at about 300° C., the stainless steel foil does not exhibit almost any dimensional change. However, if using a conventional electrolytic copper foil, the electrolytic copper foil is annealed at a temperature of about 300° C., increasingly recrystallizes, and softens, so changes in dimensions. For this reason, the laminate warps after lamination, so the precision of dimensions of the product is lowered.
In order not to cause warping in the laminate after lamination, provision of a copper alloy foil in which the dimensional change at the time of heating is as small as possible has been desired.
Further, in a TAB material, in the same way as the HDD suspension material, raising of the strength of the copper foil and lowering of the roughness of the foil surface have been demanded.
In a TAB product, a plurality of terminals of an IC chip are directly bonded to inner leads (flying leads) arranged in a device hole located at the substantial center of the product. This bonding is carried out by using a bonding system for instantaneous electrical heating and application of a constant bonding pressure. At this time, it suffers from the disadvantage that the inner leads obtained by etching of the electrolytic copper foil are pulled and stretched by the bonding pressure.
Further, if the strength of the electrolytic copper foil is low, it plastically deforms to cause slack in the inner leads. In remarkable cases, there is a possibility of breakage.
Accordingly, in order to make the line width of the inner leads finer, it is demanded that the electrolytic copper foil which is used have a roughened surface which is roughened to a low degree and have a high strength.
In this case as well, it is necessary that the copper foil have a high strength in an ordinary state (ordinary temperature/ordinary pressure state) and have a high strength even after heating. When it is used for TAB, there is used an FPC comprised of two layers or three layers formed by bonding copper foils and polyimide to each other. In a three-layer FPC, when bonding polyimide to the copper foils, an epoxy binder is used and the members are bonded to each other at a temperature of around 180° C. Further, in a two-layer FPC using a polyimide binder, bonding is carried out at a temperature of around 300° C.
Even if the electrolytic copper foil has a large mechanical strength in the ordinary state, there is no meaning if the electrolytic copper foil is softened when bonded to polyimide. The conventional high strength electrolytic copper foil has a large mechanical strength in the ordinary state and does not change much at all in mechanical strength even when it is heated at approximately 180° C. However, when it is heated at about 300° C., it is annealed and increasingly recrystallizes, therefore it is rapidly softened and falls in mechanical strength. Such copper foil is unsuitable for the purpose of TAB.
Further, copper foil is used as a current collector for use for a cell such as a lithium ion secondary cell. A lithium ion secondary cell is basically configured by a positive electrode, a negative electrode, and an electrolyte. The negative electrode is formed by coating a negative active material layer on the surface of the copper foil used as the current collector.
The method of formation of the negative electrode generally carries out by: the method of dissolving a negative active material and binder resin (added for the purpose of binding the active material and copper foil substrate) in a solvent to form a slurry, coating this on the copper foil substrate, drying it at a temperature not less than a curing temperature of the binder resin, and then pressing.
As the binder resin, polyvinylidene fluoride (PVDF), styrene butadiene rubber, and so on are widely used.
Active materials comprised of high theoretical capacity silicon-, tin-, or germanium alloy-based materials and the like have come into focus along with the increase of cell capacity in recent years. These have very large rates of expansion of volume along with insertion/desorption of lithium at the time of charging/discharging. The above-mentioned binder resins are insufficient in strength. Therefore, a polyimide resin, which has a high bonding strength with a copper substrate, has been preferably used. However, a polyimide resin has a very high curing temperature such as about 300° C. unlike the binder resin explained above, therefore a negative current collector (copper foil) durable against this heating condition has been demanded.
In this way, in both of the FPC field and secondary cell field, a polyimide resin having a very high curing temperature such as about 300° C. has begun to be used as the binder, therefore a copper foil durable against this heating condition has been demanded.
On the other hand, to obtain electrolytic copper foil which has a surface to be bonded to the polyimide resin base material with a low profile and which is excellent in mechanical strength as well, various studies such as shown below have been carried out.
For example, Patent Literature (PLT) 1, Japanese Patent No. 4120806, discloses, as a copper foil which is optimal for use for a printed circuit board and use for a negative electrode current collector for secondary cell use, a low roughened surface electrolytic copper foil which has a roughened surface which has a surface roughness Rz of 2.0 μm or less and is uniformly roughened to a low degree and which has an elongation at 180° C. of 10.0% or more.
Further, PLT 1 discloses that the above electrolytic copper foil can be obtained by using an sulfuric acid-copper sulfate aqueous solution as the electrolyte and adding polyethylene imine or its derivative, a sulfonate of active organic sulfur, and chlorine ions.
PLT 2, Japanese Patent No. 4273309, discloses an electrolytic copper foil in which the surface roughness Rz is 2.5 μm or less, the tensile strength at 25° C. measured within 20 minutes from the point of time of completion of electrodeposition is 820 MPa or more, and a rate of fall of the tensile strength at 25° C. measured when 300 minutes have passed from the point of completion of electrodeposition with respect to the tensile strength at 25° C. measured within 25 minutes from the point of completion of electrodeposition is 10% or less.
Further, PLT 2 discloses that the above electrolytic copper foil can be obtained by using an aqueous solution containing copper sulfate and sulfuric acid as the electrolyte and adding hydroxyethyl cellulose, polyethylene imine, a sulfonate of an active organic sulfur compound, acetylene glycol, and chlorine ions.
PLT 3, Japanese Patent No. 3270637, discloses a controlled, low profile electrolytic copper foil comprised of an electrolytic copper foil having a particle structure free from columnar crystals and twin boundaries and having a mean particle size of up to 10 μm wherein the particle structure is a particle structure which is substantially uniform and randomly oriented.
In this electrolytic copper foil, the maximum tensile strength at 23° C. is within a range of 87,000 to 120,000 psi (600 MPa to 827 MPa) and the ultimate elongation at 180° C. is 25,000 to 35,000 psi (172 MPa to 241 MPa).
In the case of the electrolytic copper foils disclosed in the above PLTs 1 to 3, the mechanical strength in the ordinary state is large in all instances, but the mechanical strength remarkably falls in a case where they are heated at a high temperature of 300° C. or more.
In the case of the electrolytic copper foils disclosed in the above PLTs 1 to 3, all use an electrolyte containing copper sulfate and sulfuric acid and all use organic compounds as additives though the types of additives are different (hereinafter referred to as “organic additives”).
Many organic additives usually have the effect of suppressing crystal growth. It is considered that the crystals are incorporated into the grain boundaries.
In this case, the larger the amount of the organic additive incorporated into the grain boundaries, the more improved the mechanical strength tends to become that disclosed in Non-Patent Literature (NPLT) 1: Shoji Shiga, Metal Surface Technology, Vol. 31, No. 10, p. 573 (1980)).
When the organic additive which has been incorporated into the grain boundaries is heated at a high temperature of 300° C. or more, the organic additive decomposes. As a result, it is considered that the mechanical strength is lowered.
On the other hand, as copper foil satisfying the above demands, use is made of rolled copper alloy foil. Rolled copper alloy foil is not annealed much at a temperature of about 300° C., has a small dimensional change at the time of heating, and has a small change of mechanical strength as well.
However, rolled copper foil is expensive compared with electrolytic copper foil, and it is difficult to satisfy demands on width, thickness, etc. with it.
Therefore, to obtain electrolytic copper foil which has a surface to be bonded to the polyimide resin base material with a low profile and which is excellent in mechanical strength as well, the present inventors attempted to develop an electrolytic copper alloy foil which is improved in the heat resistance of the copper foil by adding tungsten to the copper foil and is suitable for applications which use a polyimide resin as a binder resin.
However, tungsten is a metal which is very hard to incorporate into electrolytic copper foil.
PLT 4 (Japanese Patent No. 3238278) and PLT 5 (Japanese Patent Publication No. 9-67693 A1) disclose addition of tungsten to the electrolyte for forming the electrolytic copper foil.
PLTs 4 and 5 relate to copper foil for a printed circuit and disclose in their examples to form foil by an electrolyte obtained by adding 20 to 100 mg/liter of tungsten (W) and chlorine ions (chloride ions) to the electrolyte and that the formed copper foil does not have pinholes, is excellent in adhesion with the resin substrate, and has a high hot-rolling elongation at 180° C. However, there is no description that tungsten was incorporated into the copper foil, that is, that a Cu—W alloy foil was produced.
In this regard, an electrolyte containing copper sulfate and sulfuric acid is used as the electrolyte of the electrolytic copper foil. Various additives are added to the plating bath for the purpose of glossing and smoothing the copper foil surface, reducing stress of the copper foil, and so on. When no additives are used, the surface morphology, mechanical properties, etc. demanded from the copper foil are not obtained, therefore the importance of additives is very high. In particular, a copper sulfate plating bath is a simple acidic solution, therefore has a poor uniform electrodeposition property, so production of a preferred electrolytic copper foil without any additives is difficult. As additives used in the copper sulfate plating bath, chlorine ions, polyoxyethylene-based surfactants, smoothening agents, organic sulfates, or another gloss agents, glue, gelatin, etc. are proposed and used.
Unless chlorine or other additives are added to the copper sulfate plating bath, plating is concentrated at high current portions in which electricity easily flows (portions near anode, end of cathode, tip ends of pointed objects, etc.) and a generally called “burning state (where the plating surface becomes more uneven)” occurs. For this reason, about 20 to 100 mg/liter of chlorine ions are added in the usual copper sulfate plating. When the chlorine ions become less than 20 mg/liter, for the above reason, burning easily occurs. Conversely, when it exceeds 80 mg/liter, the leveling action is too strong, therefore “dull deposition” occurs in low current portions (in small holes etc.).
However, when there are chlorine ions in the electrolyte, it becomes difficult to mix a specific metal into the copper foil to change the characteristics of the copper foil. That is, with an electrolyte free from chlorine ions, it is possible to mix another metal into the copper foil. The characteristics of the copper foil can be changed by mixing (alloying) another metal, but it becomes hard to mix another metal into the copper foil if chlorine ions enter the electrolyte, and it becomes extremely difficult to change the characteristics of the copper foil by another metal.
For example, PLTs 4 and 5 disclose a method of producing an electrolytic copper foil by an electrolyte obtained by adding tungsten into a sulfuric acid-copper sulfate electrolyte and further adding glue and chlorine ions and describe, as the effect thereof, that the production of copper foil having a hot-rolling elongation at 180° C. of 3% or more and a large surface roughness, but with few pinholes is possible.
Therefore, the present inventors repeated the experiment of adding tungsten into the sulfuric acid-copper sulfate electrolyte and further adding glue and chlorine ions and could produce copper foil with the characteristics aimed at by the electrolytic copper foil disclosed in PLT 4 of a hot-rolling elongation at 180° C. of 3% or more, a large roughness of the roughened surface, and few pinholes. However, when heat treating this copper foil at 300° C. for 1 hour, it was learned that the mechanical strength could not be kept. As a result of analysis of this copper foil, it was clarified that tungsten did not form eutectoids in the electrically deposited copper.
That is, in the method of PLTs 4 and 5, the electrodeposition was carried out by an electrolyte obtained by adding tungsten to a sulfuric acid-copper sulfate electrolyte and further adding 10 mg/liter or less of glue and 20 to 100 mg/liter of chlorine ions. Therefore, an electrolytic copper alloy foil in which eutectoids of tungsten did not form in the copper foil and in which a high mechanical strength was kept even if it was heated at 300° C. could not be produced.